RFR: JDK-8299688: Adopt C++14 compatible std::bit_cast introduced in C++20 [v4]

[Justin King]

On Mon, 23 Jan 2023 08:05:54 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> src/hotspot/share/utilities/bitCast.hpp line 41:
>> 
>>> 39: // Trivial implementation when To and From are the same.
>>> 40: template <typename To, typename From, ENABLE_IF(std::is_same<From, To>::value)>
>>> 41: constexpr To bit_cast(const From& from) {
>> 
>> This is missing the tivially copiable requirement for std::bit_cast.  Also, it seems like the is_same
>> SFINAE should not be needed here.  Why not just
>> 
>> template <typename T, ENABLE_IF(std::is_trivially_copyable<T>::value)>
>> constexpr T bit_cast(const T& value) {
>>   return value;
>> }
>
> The need for this overload to match the Standard specification is one of the reasons why I had refrained
> from doing something like this change.  This overload's existence (slightly) complicates others by requiring
> an additional `!std::is_same<To, From>` constraint.  Since such a nop conversion hasn't arisen in our code
> (yet), there isn't currently a need for this.  I also don't see any tests for it.

I removed the is_same. It technically isn't needed to conform. I also decided to not conform to the full standard and restrict uses based on the existing PrimitiveConversions::cast restrictions.

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PR: https://git.openjdk.org/jdk/pull/11865